The present invention relates to components for optical communications systems, specifically multilayer optical interference filters used in interconnection and coupling devices.
Optical communications systems comprise an interconnected network of optical fibers for transmitting a plurality of the optical signal channels between nodes in the network. In order to increase the capacity of existing optical communications systems, or provide for flexible reconfiguration, multiple optical signal channels may propagate between nodes simultaneously using time division and wavelength division multiplexing (WDM).
Wavelength division multiplexing refers to a plurality of signal channels characterized by a different wavelength of light, while time division multiplexing refers to a time sequence allocation of digital signals within a common optical signal channel. Although information may be transmitted in analog format in a WDM system, the digital format is commonly used in telecommunications because of the higher data transfer rates and compatibility with time division multiplexing schemes deployed in electronic communications systems.
As a WDM communication system utilizes optical signals of different wavelengths the optical fiber network must be configured such that the time sequential nature of information traveling on different wavelengths between common nodes is not temporally distorted. While such temporal distortion is influenced by design and environmental factors, it is frequently due to the wavelength dependence of the refractive index within the optical media forming the waveguiding optical fiber. The velocity of light is decreased on transmission through a dense media, such as optical glass fibers, in proportion to the refractive index ratio between free space transmission, 1, and the refractive index of the optical glass at the signal channel wavelength ng. As a refractive index of glasses vary with wavelength, xcex, (i.e. ng=n (xcex) optical signals will be distorted, that is distributed in arrival time at the terminal node in the communication system network in proportion to the distance between originating node and the terminal node. The change in refractive index of a material with wavelength is commonly referred to as chromatic dispersion. Thus, as the distance between nodes in the optical communication system increases, or the digital pulse width decreases in order to obtain greater signal transfer capacity, the inherent properties of optical glasses become a greater limitation on performance and reliability.
Chromatic dispersion of optical fiber is roughly constant over the 1550 nm communication window, and can be compensated by several techniques including dispersion compensating fiber, which has a radial gradient in refractive index to provide self correction, Fiber Bragg gratings, etc. However, certain wavelength filtering components such as multilayer interference filters (MLIF) can have significant dispersion characteristics due to a fundamental Kramers-Kronig type relationship between transmission spectrum and dispersion characteristics. This type of temporal distortion is also quantified as the group delay (GD), typically in units of pico-seconds (ps). As the GD characteristic varies substantially with wavelength over the narrow passband (that is the high transmission region corresponding to the allocation of signal channels at specific wavelength per ITU convention) of an MLIF, the derivative of GD with respect to wavelength is also denominated or characterized as the dispersion, typically in units of ps/nm (pico-seconds/nano-meter). Such dispersion is difficult to compensate using conventional techniques such as dispersion compensating fiber.
Other sources of signal temporal distortion may arise for various active or passive components within the optical communication network, such as optical amplifiers, multiplexing filters, gain flattening filters, arrayed waveguides, Fiber Bragg gratings and the like, as well as temperature fluctuations. Accordingly, as an optical communication system is reconfigured for repair, maintenance or to meet changes in demand, the temporal distortion of signals may change in a manner that is not easily predictable. Numerous methods providing for the effects of GD, whether arising through the characteristics of the optical fiber or system components, have been developed. These methods include devices that either provide a fixed amount of compensation or an adjustable amount of compensation, or may be deployed at or between nodes in the optical communication system.
Reflective MLIF""s that provide dispersion or group delay compensation have been disclosed in U.S. Pat. Nos. 5,734,503; 6,301,049; 6,301,042; 6,222,673; 6,154,318 and 6,081,379, which are incorporated herein by reference. However, these solutions are inapplicable to a WDM system as they do not provide for single channel compensation and/or attenuate or reject the other optical signals that are not intended to be compensated. These thin film design approaches all achieve the desired group delay or dispersion control on reflection from dielectric stacks wherein the optical thickness of the quarterwave layers is perturbed to improve the group delay over the reflective stacks bandwidth. These filter designs are inapplicable not only when the modification of only a narrow bandwidth is desired, but also do not generally provide the level of GD or dispersion correction required in an optical communication system.
As new interconnections are required to insert such devices within the optical communication system it is desirable that the devices themselves, as well as the connections thereto, result in a minimum signal loss.
Accordingly, it is an objective of the invention system to provide a narrow bandpass filter that avoids the need for significant correction of group delay or dispersion correction.
It would be advantageous to provide narrow bandpass filters which achieved comparable isolation and high transmission of optical signals, while avoiding signal loss, cross-talk and other bit rate errors without compromising the ability to space optical signal channels as close as possible; that is within the limits of source laser and modulator performance.
It is a further object of the invention to provide suitable narrow bandpass filter devices are simple to fabricate, compact and thermally stable.
The inventive narrow bandpass filters have an optimized transmission profile for use in high bit rate communication systems where the variation in GD across the filter bandwidth would otherwise result in unacceptable bit error rates absent addition means for dispersion compensation.
The inventive filters deliberately do not follow the conventional practice of optimizing the bandpass region to have a xe2x80x9csquarexe2x80x9d profile with a relatively flat region of high transmission surrounding the center wavelength position of the filter. While the conventional design approach avoids signal loss arising from laser source instability, it is now appreciated that at higher bit rates the resultant xe2x80x9cnoisexe2x80x9d arising from the high dispersion in GD across the passband is dominant contributor to the bit error rate in the system.
The first object is achieved by providing a Fabry-Perot (F-P) type MLIF having a transmission and GD dispersion optimized to provide a more consistent signal to noise ration with laser or source instability such that the overall bit error rate is reduced at high bit rates. The optimized narrow bandpass filter is characterized by a high transmission at its center wavelength position and a gradually decreasing transmission across the remainder of the bandwidth. By selecting and arranging the layers of the F-P MLIF to obtain this result the wavelength variation in group delay, or dispersion, generally thought to be inherent in a narrow bandpass filter is substantially constant over the most significant region of the filters passband.
Another object of the invention is achieved by selecting and arranging the layers of the F-P MLIF to provide a 3 dB bandwidth in transmission over which the maximum ripple in group delay is determined by the optical channel spacing, 3 dB bandwidth and the bit rate of the communication system.
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.